The compounds M-2((O2CBu)-Bu-t)(4) and M-2(O2CC6H5)(4), where M = Mo or W, have been examined by femtosecond time-resolved IR (fs-TRIR) spectroscopy in tetrahydrofuran with excitation into the singlet metal-to-ligand charge-transfer ((MLCT)-M-1) band. In the region from 1500 to 1600 cm(-1), a long-lived excited state (>2 ns) has been detected for the compounds M-2((O2CBu)-Bu-t)(4) and Mo-2(O2CC6H5)(4) with an IR absorption at similar to 1540 cm(-1) assignable to the asymmetric CO2 stretch, nu(as)(CO2), of the triplet metal-metal delta-delta star ((MM)-M-3 delta delta*) state. The fs-TRIR spectra of W-2(O2CC6H5)(4) are notably different and are assigned to decay of the MLCT states. In (MM)-M-3 delta delta*, the removal of an electron from the delta orbital reduces MM delta to CO2 pi* back-bonding and causes, a shift of nu(as)(CO2) to higher energy by similar to 30-60 cm(-1) depending on the metal. TRIR spectroscopy also provides evidence for M-2((O2CBu)-Bu-t)(4), where M = Mo or W, having MM delta delta* S-1 states with nu(as)(CO2) distinct from those of the (MM)-M-3 delta delta* states.